In a diesel engine, a common-rail type fuel injection system has been known in which a common rail common to each cylinder is provided to accumulate high-pressure fuel. High-pressure fuel is force fed from a fuel supply pump to the common rail and controlled to a predetermined pressure. Injectors of the respective cylinders are then driven at predetermined timing to inject the fuel. A common-rail injector typically has a control chamber for applying a pressure in a valve-closing direction to a nozzle needle, and a control valve for controlling the pressure of the control chamber. The injector is configured so that an actuator drives the control valve to increase and decrease the pressure of the control chamber.
For the control valve, a three-way valve structure for selectively making the control chamber communicate with a high-pressure channel or a low-pressure channel is suitably used. The valve element of the three-way valve is arranged in a valve chamber provided with a low-pressure side seat leading to the low-pressure channel and a high-pressure side seat leading to the high-pressure channel. The valve element moves between the two seats to switch the seat position. With the three-way valve structure, the valve element sits on the high-pressure side seat to interrupt the communication with the high-pressure channel during fuel injection, whereby the high-pressure fuel is prevented from flowing out through the valve chamber. For example, a piezo actuator is used as the actuator. When electrically energized, the piezo actuator extends to release the valve element from the low-pressure side seat, and then sets it on the high-pressure side seat. Since the piezo actuator has excellent response, sophisticated fuel injection control is expected.
Control valves having a three-way valve structure are described, for example, in (1) Japanese Patent Laid-Open Publication No. 2000-130614; (2) Japanese Patent Laid-Open Publication No. 2002-227747; (3) Japanese Patent Laid-Open Publication No. 2001-41125; (4) Japanese PCT National Publication No. 2001-500218; and (5) Japanese Patent Laid-Open Publication No. 2001-140726. The first four patent documents listed above include a throttle disposed on the downstream side of the low-pressure side seat. This configuration advantageously suppresses the nozzle opening speed to improve controllability of the amount of injection.
Also, for the sake of operating the common-rail type fuel injection system efficiently, it is desirable to reduce fuel leakage as much as possible. Nevertheless, the first two patent documents listed above deal with a pressure balance valve, which constantly causes leakage through its sliding portion. In this case, extra work is required of the pump, and that leakage increases the fuel temperature and deteriorates the fuel. The control valve of the third patent document listed above has a spherical valve element, and in order to accommodate this, its high-pressure side seat member and low-pressure side seat member are formed as separate members. In this case, leakage can occur due to positional shifts of the two members. This is described in the second and fifth patent documents listed above. Therefore, this is difficult to use when the amount of lift is small.
The fifth patent document listed above proposes that a plurality of valve members capable of relative movement be arranged so as to allow proper operation even with positional shifts. However, this configuration gets very complicated. Moreover, for improved controllability on the amount of injection, it is desirable to increase the nozzle closing speed. In general, the opening area of the high-pressure side seat can be increased to increase the nozzle closing speed. Nevertheless, since the piezo actuator has the characteristic that the displacement and the produced force are inversely proportional to each other, the increased opening area of the high-pressure side seat makes the closing driving force greater, thereby causing the problem of reduced energy efficiency.